This invention relates to methods of manufacturing electronic devices comprising a thin-film circuit element, including the step of directing an energy beam at a surface of a semiconductor thin film to crystallise at least a portion of the thin film. The device may be a flat panel display (for example, a liquid crystal display), or a large area image sensor or several other types of large-area electronic device (for example, a thin-film data store or memory device). The invention also relates to apparatus for crystallising a portion of a semiconductor thin film.
There is much interest in developing thin-film circuits with thin-film transistors (hereinafter termed TFTs) and/or other semiconductor circuit elements on insulating substrates for large-area electronics applications. These circuit elements fabricated with portions of an amorphous or polycrystalline semiconductor film may form the switching elements in a cell matrix, for example in a flat panel display as described in U.S. Pat. No. 5,130,829 (our reference PHB 33646), the whole contents of which are hereby incorporated herein as reference material.
Recent developments involve the fabrication and integration of thin-film circuits (usually with polycrystalline silicon) as, for example, integrated drive circuits for such a cell matrix. In order to increase the circuit speed, it is advantageous to use semiconductor material of good crystal quality and high mobility for thin-film islands of the TFTs of these circuits. It is known to deposit a semiconductor thin film of amorphous material or of low crystallinity material and then to form the material of high crystallinity in at least a device portion of this film by exposure to an energy beam from a laser.
U.S. Pat. No. 5,372,836 discloses a method of manufacturing an electronic device comprising a thin-film circuit element, which method includes the steps of:
(a) directing an energy beam at a surface area of a semiconductor thin film on a substrate to crystallise at least a portion of the thin film, PA1 (b) monitoring the surface quality of the crystallised portion of the thin film by directing light at the surface area of the crystallised portion and by detecting with a light detector the light returned from the surface area, the light detector giving an output indicative of the monitored surface quality, and PA1 (c) setting the energy of the beam in accordance with the output from the light detector to regulate the crystallisation of a device portion of a semiconductor thin film at which the beam is subsequently directed with its set energy. PA1 (a) directing an energy beam at a surface area of a semiconductor thin film on a substrate to crystallise at least a portion of the thin film, PA1 (b) monitoring the surface quality of the crystallised portion of the thin film by directing light at the surface area of the crystallised portion and by detecting with a light detector the light returned by the surface area, the light detector giving an output indicative of the monitored surface quality, and PA1 (c) setting the energy of the beam in accordance with the output from the light detector to regulate the crystallisation of a device portion of a semiconductor thin film at which the beam is subsequently directed with its set energy, PA1 characterised in that the light detector is located at a position outside the specular reflection path of the light returned by the surface area of the crystallised portion and detects a threshold increase in intensity of the light being scattered by the surface area of the crystallised portion, which threshold increase occurs when the energy of the beam is increased sufficiently to cause an onset of surface roughening, and in that, when crystallising the device portion for the thin-film circuit element during the step (c), the energy of the beam is set to a value as determined by the detection of said threshold increase. PA1 a laser for generating an energy beam to crystallise the portion of the thin film, PA1 a processing cell containing a support for mounting the substrate, PA1 an optical system between the laser and the processing cell to direct the beam from the laser at a surface area of the thin film when the substrate is mounted in the processing cell, PA1 adaptor means for changing the energy of the beam incident on the film, PA1 a light source for directing light at the surface area of the crystallised portion of the thin film, and PA1 a light detector for detecting the light returned by the surface area, the light detector giving an output indicative of the surface quality, PA1 characterised in that the light detector is located at a position outside the specular reflection path of the light returned by the surface area of the crystallised portion and has sufficient sensitivity to detect a threshold increase in intensity of the light being scattered by the surface area of the crystallised portion, which threshold increase occurs when the energy of the beam is increased sufficiently to cause the onset of surface roughening, and control means take an input from the output of the light detector and provide an output to the adaptor means for setting the energy of the beam to a value as determined by the detection of said threshold increase.
The whole contents of U.S. Pat. No. 5,372,836 are hereby incorporated herein as reference material.
In the method and apparatus described in U.S. Pat. No. 5,372,836 the light detector is a spectroscope 16. The light source 17 for directing the light at the surface area of the crystallised thin-film portion has a wide wavelength band from 200 nm to 500 nm. The spectroscope 16 is located in the specular reflection path of the light returned by the surface area of the film. Sample outputs of the spectroscope 16 are depicted as graphs in FIGS. 18 to 21 of U.S. Pat. No. 5,372,836, showing the bandgap spectral reflectance of a film in various crystallisation states. The spectral reflectance shown in these graphs are for a polycrystalline silicon film in an ideal state in FIG. 18, for an amorphous silicon film in FIG. 19, for an amorphous silicon film insufficiently transformed into polycrystalline silicon in FIG. 20 due to an insufficient energy of the laser beam, and for a film damaged by an excessive energy of the laser beam in FIG. 21.
In the method described in U.S. Pat. No. 5,372,836, the semiconductor thin film is deposited on the substrate as hydrogenated amorphous silicon material by a plasma CVD (chemical vapour deposition) process. The device portion of the film is subjected to multiple exposures with the laser beam, the energy of the laser beam being progressively increased with each exposure. Initially, the energy levels of the laser beam are set such that hydrogen is gradually discharged from the film without crystallising or damaging the film. The energy of the beam is finally set such that the film is transformed into a polycrystalline silicon material. The spectroscope 16 provides a good source of information on the quality of the exposed film portion at the different stages.
As can be seen from FIGS. 18 to 20 of U.S. Pat. No. 5,372,836, such an arrangement with specular reflection to a spectroscope can provide a good indication of whether the film is still inadequately crystallised so enabling increased crystallisation of the film to be carried out by a further exposure with an increased beam energy. As shown in FIG. 21, such an arrangement is also good for detecting when an excessive energy level has been used and has damaged the film. However, as described in column 13, lines 12 to 15, when the spectroscopic reflectance distribution shown in FIG. 21 is detected, the sample is a defective one. It is then too late to remedy the situation, and the sample can only be discarded as being defective.